The present invention generally relates to a picture element data generating method, and in particular to a picture element data generating method in which gradation levels and a number of picture elements can be expanded on the basis of image data composed of small pixels each represented by small gradation levels. The present invention is suited for image data transmission utilizing a narrow frequency band range transmission line such as a public telephone line. The present invention may be applied to a gradation facsimile system, a video conference system, a video telephone system, a videotex or the like.
Recently, image (or picture) transmission has been widely used as a method of data communications. In particular, facsimile communications have become very popular. As well known, a conventional facsimile system is designed so that on a transmitting side, image data read out by a sensor are encoded into a binary level image signal and transmitted, and on a receiving side, the binary level image signal is decoded to the original image data, which is then printed.
Presently, with the progress of image processing techniques, many kinds of image transmission systems other than the conventional facsimile system which handles the binary level image signal, are turned to for practical use. Examples of these are gradation facsimile systems, video conference system, a video phone and a videotex. For example, the gradation facsimile system can transmit a half-tone image other than white and black through a line. That is, the gradation facsimile system converts the image data from the sensor into a multilevel image signal. Also, the video phone can transmit a still picture composed of multilevel image data. As will be anticipated from the above description, the above image transmission systems process a large amount of image data, compared with the conventional facsimile system which processes the binary image data.
Currently, most of these image transmission systems are designed for use on a leased line or a high bit rate line. In other words, these lines have bit rates higher than 9,600 bps (bits per second), which amounts to the bit rate of the public telephone line in Japan. For example, an image transmission system applicable for the video phone based on the leased line having a bit rate of more than 9,600 bps is disclosed in Sato et al, "IMAGE TRANSMISSIONS AND APPARATUSES THEREFOR", Colona Company, 1977. Also, a video conference system and a video phone system are disclosed in H. Yasuda, "WIDE BAND RANGE IMAGE COMMUNICATIONS", Journal of the Institute of Television Engineers of Japan, vol.40, No.7, 1986, pp.655-658. The systems disclosed are predicated on use of the leased line having the bit rate more than 9,600 bps. In other words, these transmission systems are based on a transmission channel of a relatively wide frequency band range.
It is desirable to transmit the image data processed in the video phone system, video conference system or gradation facsimile system through the public telephone line. As mentioned above, the public telephone line employed in Japan has a bit rate of 9,600 bps and is therefore a narrow frequency band range transmission line (in the frequency range of 0.3 to 3.4 kHz). It is now assumed that one frame of a video signal is formed with 512.times.512 picture elements (hereafter referred to as pixels) and that one pixel is represented with 32 gradation levels (corresponding to 5 bits). When the video signal is a color video signal and is transmitted through the public telephone line having a bit rate of 9,600 bps, time required for transmission of one frame of the color video signal is obtained as follows: EQU 3.times.5.times.512.times.512/ 9600 =409.6 (sec).
Therefore, the image transmission under the above condition is not suited for practical use.
For this reason, compression of the image data is generally employed to transmit the image data with high speed. For example, a modified Huffman coding scheme and a modified read coding scheme are widely employed for the compression of the binary image data in the facsimile system. Also, a bit plane coding scheme, a level plane coding scheme and a Markov coding scheme are employed for the compression multilevel image data in the video system, for example.
However, as clearly shown in FIG. 1, the degree of the data compression becomes smaller as the gradation levels (gray levels), or the number of bits representing one pixel increases. Therefore, in addition to the conventional compression schemes, there is used another data compressing method in which part of original image data are intentionally omitted and then compressed in order to facilitate the reduction of the quantity of image data to be transmitted. That is, all of the sampled image data are not used for transmission, but some of them are intermittently extracted from the original sampled image data. For example, when the sampled data at the even numbered sampling times are extracted from all the sampled image data and then transmitted without being compressed, the quantity of the image data to be transmitted becomes half of the quantity of the original image data. In other words, the transmission speed is doubled. In addition, the image data intermittently extracted from the original sampled data is compressed, and the data quantity to be transmitted is further reduced. For example, when one frame of the color video signal having the same size as one frame in the above example (512.times.512; 32gradation levels) is sent with 64.times.64pixels and 8gradation levels, a time required for the transmission becomes as follows: EQU 3.times.3.times.64.times.64/ 9600 =3.84(sec).
In response to the reduction of the data quantity, it becomes necessary to accurately reproduce the original image from the intermittent image data to obtain a high definition image on the receiving side. Therefore, a pixel data generating method for increasing pixels is used to restore pixel data which are omitted on the transmitting side by means of an interpolation process or the like.
One conventional pixel data generating method is disclosed in the Japanese Laid-Open Pat. Publication No.97,774/1985. This publication discloses an increase of the number of pixel data in which pixels are simply increased without any signal processing. Thereafter, a known dither process is applied to increase pixel data. Although this method can simply increase pixel data, the picture quality of the reproduced image may be affected. Actually, the picture quality of the image reproduced by the above pixel increasing method is substantially the same as that of the image reproduced without using this method.
Another conventional pixel data generating method is proposed in which levels of mutually adjacent pixels are simply averaged and pixels having the averaged level are interpolated, so that the number of pixels is increased. This averaging method can provide a picture quality better than the picture quality provided by the method disclosed in the above publication. However, the averaging method expands only the number of pixels and does not expand the gradation levels. For this reason, it is difficult to obtain a reproduced image of high picture quality. Moreover, it is difficult to obtain a high resolution in the reproduced image in which characters and graphics are mixed.